200 more watts for probably a similar weight (both extremely heavy though). They both have relatively simple controls. If the wattage doesn't matter, then it just comes down to tone. Ampeg and Mesa both are experienced in making great tube amps. I prefer Mesa, and the extra watts, but that's just me

They're both 300 watt tube amps. Mesa's 500 watt rating is for peak and is advertising hype. The Mesa has a more laid back tone, while the Ampeg will be more aggressive sounding. This has been covered about 4000 times before. Search for past posts on the subject.

I did a bit of research on tube bass amps a couple of years ago and this is what I found to be some differences between the 2 bass heads.
The Ampeg uses 6x6550 power tubes and the Mesa uses 12 x 6L6's. 6550's put out about 100 watts per pr and 6L6's about 50 watts per pr. So really they are both 300 watt tube amps. It is my understanding that there is no bias adjustments for the Mesa. They are setup to work with Mesa's tubes and you replace what you need with matching ref numbers. Ampeg on the other hand has adjustable bias for tubes with some sort of led deal so you probably have more latitude with tube replacement (Psycho, am I right on this one?) They both have their own idea of how bass should sound. Ampeg = bass in the forfront and Mesa = bass in the background. (to my ears) You'll have to decide for yourself which sound you prefer.

The mesa 400 (not 400+) has an ajustable bias control, so I assume that the 400+ does as well, but I could be very wrong. I have to disagree with the bass in the forefront/backround statement though. The mesa has a fat, punchy sound that can stand out just as well as the ampeg.

The mesa 400 (not 400+) has an ajustable bias control, so I assume that the 400+ does as well, but I could be very wrong. I have to disagree with the bass in the forefront/backround statement though. The mesa has a fat, punchy sound that can stand out just as well as the ampeg.

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From the manual:POWER TUBESThe Bass 400 Plus uses twelve MESA 5881 STR-425 power tubes. This massive power
section places very little strain on any individual tube, which allows for tremendous
headroom and greater tube reliability (you'll notice how cool they run). You can expect
about two years of outstanding performance from the tubes; when replacements are
needed, be sure to use only genuine MESA 5881's (available from MESA/Boogie Pro
Centers and Tube Dealers, and from MESA/Boogie directly). The use of other tube
brands will invalidate your Bass 400 Plus warranty! Amplifier bias has been permanently
set at the factory and should never need adjustment.

This is what I based my statement about the bias adjustments on. I shouldn't have assumed there were no adjustments which there probably are. Mesa just wants you to always buy their tubes etc. etc.